You can accomplish this in the main document via a Regular Expression as follows:
Using Search and Replace (Ctrl+H)
In the Find box: ([0-9])(-)([0-9])
In the Replace box: \1--\3 (Note: I used -- to denote the en-dash but you would enter the correct character by holding Ctrl and pressing the minus on the Number Key pad!)
Also make sure that the Use Wild Cards option is checked and that Search: is set to ALL.

Also NOTE: This will not with any numbers that are displayed via Field Codes like page numbers, number of pages, figure numbers, etc.

How it works:The find box:
The parens () set up groups, in this case 3 groups (first number) (hyphen/dash) (second number)
The [] setups a range and will match any one 1 thus [0-9] will select any single digit numberThe replace box:
We tell it to keep the first number by using \1 {group 1}
We tell it to replace the hyphen/dash by typing the code for the en-dash
We tell it to keep the 2 number by using the \3 {group 3 in the Find box}

The en dash may be an available option via the Special pulldown in the Find and Replace "More>>" dialog—and if it is, will appear in the replace with box as the ^= token (and ^+ for the longer em dash).

RetiredGeek's Ctrl-NumericPadHyphen entry method will work for the en dash (in Windows; Option-Minus on a Mac), but the result can be hard to differentiate from a hyphen in a dialog box. An alternative is to use ^0150 to enter the en dash character (and ^0151 for an em dash).